The invention relates to the structure, manufacture and use of synthetic implants, e.g., a corneal prosthesis.
A synthetic implant is a synthetic material member incorporated into a living body, typically replacing or assisting a failing living component. To be successful it must serve its intended purpose and not be rejected by the receiving body or otherwise have unacceptable side effects.
A corneal prosthesis, often referred to as a keratoprosthesis, replaces part or all of the cornea, typically when the cornea has been damaged so as to cease serving the function of an optically transparent window to the retina. A complication to be avoided with a corneal prosthesis is extrusion of the prosthesis from the eye when epithelial tissue grows in behind the prosthesis.
Two types of corneal prostheses employ a threaded optical element (e.g., methyl methacrylate). In one, typically referred to as a stromal or intralamellar fixated keratoprosthesis, a threaded ring portion is embedded in the cornea, and a threaded optical shaft passes through it. In the second, typically referred to as a nut and bolt design, a threaded optical shaft has a mushroom-shaped cap, and a threaded "nut" is placed behind the cornea. Examples are described in Barnham, J. J., et al., "Keratoprosthesis: a long-term review", British J. of Ophthalmology, 1983, Vol. 67, pp. 468-474; Cardona, H., "Prosthokeratoplasty", Cornea, 1983, Vol. 2, pp. 179-183; Peyman U.S. Pat. No. 4,470,159; and Binder U.S. Pat. No. 4,586,929. Cardona describes securing Dacron mesh covered with autologous tissue to the ring to promote tissue ingrowth to retain the prosthesis. Binder discloses using hydrogel material having a water content between 30% and 79% for the ring and possibly also the optical cylinder, reporting avoidance of extrusion of the hydrogel material from the eyes.
Some other corneal prostheses are disclosed in White U.S. Pat. No. 4,612,012; Kelman U.S. Pat. No. 4,563,779 and Kern U.S. Pat. No. 4,676,790. The White prosthesis employs a centrally lenticular disc adhesively secured to a ring having an outer tissue contacting surface of a biologically compatible material. Kelman discloses removing a corneal plug from an eye, boring from the posterior surface toward the anterior surface but leaving the Bowman's lamina and epithelial layer intact, providing an optically clear plug in the bore, and returning the corneal plug to the eye. Kern discloses making a recess partially into the cornea, and bonding in the recess a shaped lens of cross-linked collagen or collagen-like material; the anterior surface has a protein condensate (formed, e.g., during laser shaping of a lens blank) that simulates a Bowman's lamina and permits growth of epithelial cells over the top of the lens implant.